The invention relates to an implant for fixing two bone fragments to each other, in particular for fixing an axially corrected capitulum of a metatarsal bone, e.g. hallux valgus, which implant comprises a clasp with two arms which are connected to each other at one of their ends and in this area form an eyelet with an opening for passage of a screw which can be screwed into one of the two bone fragments, and which, with their other, free ends, can be introduced into the other bone fragment.
Osteotomies for treating hallux valgus have already been known for decades and have the object of functionally reconstructing the axis of the 1st metatarsal. It is necessary in this context, after the osteotomy, to fix the two bone fragments in their corrected position in order to prevent mobility between the fragments and to permit reliable osseous union without dislocation.
To fix the osteotomy, it is already known to use plates which are secured to the cortical bone by means of a number of screws in order to prevent the bone fragments from buckling. A considerable additional surgical effort is required for this.
Bone clamps are also known for fixing the osteotomy, but their use involves the risk of splintering of the bone.
AT 000 937 U has disclosed an implant for fixing two bone fragments to each other, in particular for treating an axial deviation of a metatarsal bone, e.g. hallux valgus, which implant comprises a clasp with two arms which are connected to each other at one of their ends and in this area delimit an opening for passage of a screw which can be screwed into one of the two bone fragments and which, with their other, free ends, can be introduced into the medullary cavity of the other bone fragment and can spread apart. After osteotomy has been performed, the clasp is introduced proximally into the medullary cavity of one bone fragment via the free ends of the two arms and spreads apart in this medullary cavity, after which the clasp is anchored on the other bone fragment by means of the screw guided through the opening.
As a result of the spreading force and the frictional fit of the clasp arms, and as a result of the intermittent compression arising upon functional loading, the intramedullary part of this known implant has a high degree of stability. By contrast, however, the extramedullary part is anchored in the bone via only one screw, whereas a two-point bearing is required on account of the torque which increasingly occurs as the compression of the screw anchoring between implant and metatarsal head decreases.
In order to improve and stabilize the fixing of this known implant to the bone fragment which is connected to the implant via the screw, it is known to provide, between the clasp opening and the screw head, a shim part through which the screw passes and which has projections which can be anchored in the bone fragment (WO 00/06036). Since this shim part is designed as a separate element, additional difficulties arise during implantation, and additional instruments are required.
The object of the invention is to make available an implant which is easier to implant and which, when implanted, ensures greater stability.
The solution according to the invention is for the implant to have a guide bushing for the screw, which guide bushing is inserted into the clasp eyelet and is secured thereon, its inner surface corresponding substantially to the outer surface of the screw in the area adjoining the screw head, and its minimum internal diameter being at least as great as the maximum diameter of the screw thread.
Thus, a shim part is no longer provided, but instead a guide bushing which is firmly secured on the clasp. In this way, implantation is easier, and no additional instruments are necessary. The invention, however, goes beyond this. Specifically, the guide bushing results in increased stability because the screw, in the implanted state, is fitted into said guide bushing in a manner substantially free of play, so that the angle stability between clasp and screw is ensured.
The inner surface of the guide bushing could, for example, have a slightly conical configuration, in which case the outer surface of the screw in the area adjoining the screw head is then of corresponding configuration so that the screw can be arranged substantially free of play in the guide bushing. In a particularly advantageous embodiment, however, the inner surface of the guide bushing and the corresponding outer surface of the screw are substantially cylindrical.
Another embodiment according to the invention is characterized by the fact that the inner surface of the guide bushing has a thread adapted to the outer surface of the screw.
In an advantageous embodiment, the guide bushing is secured on the clasp eyelet by having an edge area engaging at least partially around the clasp eyelet. In this case, it can be secured permanently on the clasp eyelet by turning the edge area back. The guide bushing can in this case have an annular recess at least partially receiving the clasp eyelet.
The surface with which the implant bears on the bone fragment connected to the implant via the screw must be offset in relation to the plane of the arms. In the prior art, this is achieved by the fact that the ends of the arms forming the clasp eyelet lie substantially in a plane which is parallel and offset in relation to the plane of the arms in which the remaining part of the arms lies. This can also be provided for in the implant according to the invention, but is not necessary, because the surface with which the implant bears on the bone fragment connected to the screw is laterally offset on account of the axial length of the guide sleeve when the guide sleeve extends axially from the clasp eyelet to the bearing surface. In any case, however, the bend of the area of the clasp eyelet can be made smaller, which has the advantage that the material here is not so strongly stressed upon bending and consequently there is much less risk of material breaks occurring in this area. However, the lateral offset of the clasp eyelet which can nevertheless be provided for has the advantage that in this case the screw does not project beyond the plane of the arms.
The inner surface of the guide bushing advantageously has a length of at least 3 mm in the axial direction.